1. Field of the Invention
This invention relates to fuel cells and more particularly to a plurality of fuel cells connected electrically in series in a stack.
2. Description of the Prior Art
A basic fuel cell comprises an electrode spaced apart from a cathode electrode with an electrolyte disposed therebetween in a compartment formed between the two electrodes; each electrode also includes a catalyst layer on the electrolyte side thereof. On the nonelectrolyte side of the anode electrode is a reactant gas chamber for carrying a fuel, and on the nonelectrolyte side of the cathode electrode is a reactant gas chamber for carrying an oxidant. The electrodes are constructed so that the gas diffuses therethrough and comes into contact with the electrolyte in the catalyst layer thereby causing an electrochemical reaction whereby ions travel from the cathode electrode through the electrolyte to the anode electrode. This flow of ions is basically the electric current produced by the cell. In a fuel cell power plant a plurality of fuel cells are connected electrically in series through plates separating adjacent cells, thereby forming a stack. These plates, in combination with the electrodes adjacent thereto, generally define the reactant gas passages hereinbefore referred to. The voltage across the stack is the sum of the voltage drops across the individual cells, which is a function of the current produced by each cell. The amount of current produced by each cell is directly proportional to the amount of reactant gas utilized in the electrochemical reaction.
In one form of the prior art the fuel passes only once in parallel through the cells in the stack, entering at one side, traveling straight through and exiting at the other side. If one of the cells in the stack has a maldistribution of current, such as may occur due to an uneven catalyst layer, then a certain area of that cell is not passing its fair share of current. The remaining portion of that cell must now carry the entire current. This remaining portion of the cell may, for example, be carrying only 80% of the fuel flowing through the cell, but it now must support 100% of the current. If there is not enough fuel to support the current the cell begins to burn the structural components, sometimes resulting in cell or stack failure. This not only occurs in the individual cell which has the maldistribution, but it also occurs in several cells immediately downstream (in terms of current flow) from the bad cell.
One solution has been to flow the fuel through the cells in serpentine fashion such that, in theory, the entire mass of fuel entering the cell passes over every portion of the electrode. Now, even though only, say 80% of the cell is carrying 100% of the current, 100% of the fuel passes over the 80% of the cell and is available to support 100% of the current.
The serpentine cell design is not, however, satisfactory for a cell having a blockage in a fuel chamber thereby preventing the flow of fuel therethrough. That cell will necessarily be deficient in hydrogen wherein structural corrosion and ultimate cell and/or stack failure may occur. Also, a serpentine flow path results in maldistribution of the reactant gas to certain areas of the cell causing flow discontinuities, such as result when the gas travels around corners of the serpentine path. This reduces fuel utilization.